Method for determining automatically a FEXT/NEXT transfer-function

ABSTRACT

A method of automatically determining a far-end crosstalk (FEXT) and near-end crosstalk (NEXT) transfer function in communication lines such as Digital Subscriber Lines (DSL). In a first phase, an input test signal with a known power density spectrum (PSD) covering a frequency range of interest is transmitted at the near end of line A while a received signal or noise-related quantity or PSD is measured at both the near end and far end of line B. In a second phase, transmission of the test signal is stopped, and a received signal or noise-related quantity or PSD is again measured at both ends of line B. In a third phase, the FEXT/NEXT transfer function is determined based on the measurements of the first and second phases.

TECHNICAL FIELD

The invention relates to a method for determining automatically aFEXT/NEXT transfer function in communication lines used for datatransmission such as DSL (digital subscriber lines).

BACKGROUND

DSL systems have gained an ever growing importance in digitalcommunication and data transfer as the use of internet is expanding toall area of business and home applications. With the introduction of newapplications requiring the transmission of a great amount of data ademand for broadband transmission has emerged. Transmission rate andreliability have increased in recent years and have to be improvedfurther in order to comply with future requirements of digitalcommunication systems. Signal to Noise Ratio (SNR) on communicationlines is one of the factors that influence broadband capabilities.Crosstalk is usually a major noise generating factor in modern DSLsystems such as ADSL and VDSL.

Due to for instance the imperfections in the cable, signals on one linecan couple to another line resulting in an increased noise in thereceived signal on this line. This crosstalk between lines will when thesignals are transmitted in the same direction result in far-endcrosstalk (FEXT). Crosstalk signals coupled in from a transmitter to areceiver at the same side is called near-end crosstalk (NEXT). This isillustrated in FIG. 2.

For improving the broadband capabilities of communication cables andincreasing the transmission rates in both downstream and upstreamdirections, it is vital to reduce the transmit power in order reduce theFEXT and NEXT which are the main noise generating factors. There havebeen many attempts for the mitigation of crosstalk. As crosstalkcancellation can not be done in each scenario, dynamic spectrummanagement (DSM) is an effective way of eliminating the negative impactof crosstalk.

Traditional spectrum management is done in a static way by StaticSpectrum Management (SSM). Based on standardized methods to describeDSL-channels the spectral power distribution is setup once in theinitialization phase of service provisioning. This setup is then keptduring the complete service session. Dynamic spectrum management (DSM)tries to utilize and track the present channel conditions in order toexploit the transmission capacities in an optimum way. The basicalgorithms are described by Wei Yu, George Ginis and John M. Cioffi:“Distributed Multiuser Power Control for Digital Subscriber Lines (IEEEJournal on Selected Areas in Communications, Special Issue on TwistedPair Transmission, vol. 20, no. 5, pp. 1105-1115. June 2002) and byRaphael Cendrillon and Marc Moonen in “Iterative Spectrum Balancing forDigital Subscriber Lines (IEEE International Communications Conference(ICC), Seoul, May 2005). Raphael Cendrillon and Marc Moonen, JanVerlinden and Tom Bostoen, Wei Yu have described “Optimal SpectrumBalancing for Digital Subscriber Lines (IEEE Transactions onCommunications, pages 922-933, vol. 54, no. 5, May 2006.) By smartadjustments of the transmission parameters the system rate, reach andtherefore coverage can be substantially increased. Other possibleenhancements are higher line/system robustness by providing largerSNR-margins. Depending on the DSM-algorithm used different channelinformation is needed. For most cases the magnitude of the FEXT-transferfunction is sufficient but also crucial.

In order to make use of a DSM algorithm for the reduction of crosstalk,the FEXT/NEXT transfer function has to be determined in an automatic,time and cost efficient way.

Traditionally the FEXT/NEXT transfer function will be determined byusing dedicated devices (a signal generator and a network analyzer) onboth cable ends. This is easily done as long the cable is on a drum andthe person carrying out the measurement has access to both cable ends.In practical cases however, the cable is very long and the cable endsare not accessible either. One cable end is typically in a centraloffice (CO) and the other cable end is at the user side connected to acustomer premises equipment (CPE). When performing a traditionaltransfer function measurement, both cable ends have to be disconnectedfrom the DSL modems and connected to the dedicated devices. Duringmeasuring, communication on the tested lines or disconnected cable willnot be possible. All steps of measurement have to be controlled andsynchronized from either side in order to obtain the required transferfunction. An additional difficulty arises from the fact that theuser/customer equipments are distributed in different rooms, buildings,streets, districts or cities. Therefore only very few lines can bemeasured in a cost efficient way. Measuring on many or all lines wouldrequire a very long time which makes this approach practically notfeasible.

It is commonly known that operators have no access to FEXT/NEXT-transferfunction information about their installed cable binders. This is quitenatural, since measurements of these qualities can not be doneautomatically, according to our knowledge. Hence, they can only be mademanually. However, this is extremely expensive because it

-   -   is time consuming    -   requires an educated technician with suitable equipment    -   needs physical access to lines on customer side in addition to        the central office    -   is non-automatic    -   does not allow to track changes in the transfer function over        time and deployment    -   involves complicated maintenance of storing information        (database handling).

U.S. Pat. No. 6,205,220 suggest a method and apparatus for reducing thenear-far crosstalk interference between channels in a communicationsystem. Channels of different lengths that are disposed adjacent to eachother and carrying signals at the same frequencies often createcross-talk interference in their neighbouring channels. By spectrallyshaping the signals carried on shorter lines the amount of cross-talkinterference generated by these lines to longer lines may besignificantly reduced, resulting in a better overall performance. Thismay be accomplished by including spectrally shaping a signal carried ona first channel to reduce the amount of cross-talk coupling to aneighbouring second channel. The shaping of the signal carried on thefirst channel may also be based in part on characteristics of the secondand/or first channel. The characteristics include for example the lengthof the channels and the transfer functions of the channels. This methodconcentrates on the problem of non-uniform far-end crosstalk, oftenreferred to as “near-far FEXT” or “unequal-level FEXT” and makes use ofDSM on the basis of transfer functions but the way of determining thetransfer function is not addressed.

WO2005/114861 suggests the use of operational data to determine the FEXTinterference induced by one line into the other DSL line. FEXTinterference can be calculated using the NEXT interference measuredbetween the two lines at the upstream ends of the loops and thedownstream channel transfer function of one of the loops. Because theNEXT and transfer function constitute a linear time-invariant system, asdoes the FEXT interference between the lines, the NEXT interference andline transfer function can be multiplied (if in linear format) or added(if in logarithmic format) to approximate the FEXT interference betweenthe lines. This method does not require the lines to be disconnected orthe normal operation to be interrupted during measurement, but does notprovide a direct measurement of FEXT transfer function, therefore themeasured parameters provide only an estimated parameter of low accuracy.

Therefore it is an object of the invention to provide a method fordetermining the FEXT/NEXT transfer function automatically in a time andcost efficient way. A further object of the invention is to provide amethod that makes it possible to determine the FEXT/NEXT transferfunction by means of using for example the standardized Loop diagnosticprotocol in ITU-T G.992.3/5. It is also an object of the invention toprovide a measuring method that eliminates the need for use of dedicateddevices.

SUMMARY OF THE INVENTION

According to a first aspect of the invention the above objects may beachieved by a method comprising the steps of

-   -   a) transmitting a test signal with a known power spectrum        density (PSD or P_(A)) covering the frequency range of interest        on line A at least during measuring intervals,    -   b) measuring a first received signal or a noise related quantity        at a first or near end of line B and sending a first report of        the first received signal or PSD to a central control unit,    -   c) measuring a second received signal or a noise related        quantity at a second or far end of line B and sending a second        report of the second received signal or PSD to a central control        unit,    -   d) stopping transmission of the test signal on line A,    -   e) measuring a third received signal or noise related quantity        at a first or near end of line B and sending a third report of        the third received signal or PSD to a central control unit,    -   f) measuring a fourth received signal or noise related quantity        at a second or far end of line B and sending a fourth report of        the fourth received signal or PSD to a central control unit, and    -   g) determining the FEXT/NEXT transfer function on the basis of        the reported data on the received signals at the central control        unit wherein all steps are coordinated and controlled from a        central location at the central office or customer side.

By using this method, the measuring steps for determining the NEXT/FEXTtransfer function may be accomplished in a fully automated way. Thismeasurement process is controlled and coordinated by a central controlunit, therefore no human personal is needed. The speed of obtaining theresult of the measurement depends only on the measuring cycles and thedata transfer rate in the reporting cycles. The calculation cycles areperformed by the central control unit providing very short calculationcycles relative to the measuring and reporting cycles therefore it maybe neglected. DSL modems with Double Ended Line Test (DELT)functionality on the central office (CO) and user or customer premisesequipment CPE side will provide the possibility of generating andmeasuring a test signal and sending a report on the measuring result tothe central control unit, which eliminates the need for any dedicateddevice.

In another aspect of the invention the method may be accomplished bymeasuring noise related quantities, wherein

step b) comprises the measuring of PSD QLNNE_(B) ^(Phase1) at a first ornear end of line B,

step c) comprises the measuring of PSD QLNFE_(B) ^(Phase1) at a secondor far end of line B,

step e) comprises the measuring of PSD QLNNE_(B) ^(Phase2) at a first ornear end of line B,

step f) comprises the measuring of PSD QLNFE_(B) ^(Phase2) at a secondor far end of line B, and

step g) comprises determining FEXT transfer function according to

$\begin{matrix}{{H_{{AB}\;}}^{2} = \frac{{QLNFE}_{B}^{{Phase}\; 1} - {QLNFE}_{B}^{{Phase}\; 2}}{P_{A}}} & (1)\end{matrix}$and determining NEXT transfer function according to

$\begin{matrix}{{G_{AB}}^{2} = \frac{{QLNNE}_{B}^{{Phase}\; 1} - {QLNNE}_{B}^{{Phase}\; 2}}{P_{A}\;}} & (2)\end{matrix}$where P_(A) is the known power spectral density (PSD) on line A.

According to a second aspect of the invention the method may beaccomplished by measuring other noise related quantities, in which

step b) comprises the measuring of SNR SNRNE_(B) ^(Phase1) at a first ornear end of line B,

step c) comprises the measuring of SNR SNRFE_(B) ^(Phase1) at a secondor far end of line B,

step e) comprises the measuring of SNR SNRNE_(B) ^(Phase2) and a noisesignal or PSD σ_(NE-B,eff) ²(f) at a first or near end of line B,

step f) comprises the measuring of SNR SNRFE_(B) ^(Phase2) and a noisesignal or PSD σ_(FE-B,eff) ²(f) at a second or far end of line B, and

step g) comprises determining FEXT transfer function according to

$\begin{matrix}{{{H_{AB}(f)}}^{2} = {\frac{1}{P_{A}(f)} \cdot \lbrack {{{\sigma_{{{FE} - B},{eff}}^{2}(f)} \cdot \frac{{SNRFE}_{B}^{{Phase}\; 2}(f)}{{SNRFE}_{B}^{{Phase}\; 1}(f)}} - {\sigma_{{{FE} - B},{eff}}^{2}(f)}} \rbrack}} & (3) \\{{{G_{AB}(f)}}^{2} = {\frac{1}{P_{A}(f)} \cdot \lbrack {{{\sigma_{{{NE} - B},{eff}}^{2}(f)} \cdot \frac{{SNRNE}_{B}^{{Phase}\; 2}(f)}{{SNRNE}_{B}^{{Phase}\; 1}(f)}} - \sigma_{{{NE} - B},{eff}}^{2}} \rbrack}} & (4)\end{matrix}$

According to a third aspect of the invention the method may beaccomplished by measuring other noise related quantities, wherein

step b) comprises the measuring of SNR SNRNE_(B) ^(Phase1) at a first ornear end of line B,

step c) comprises the measuring of SNR SNRFE_(B) ^(Phase1) at a secondor far end of line B,

step e) comprises the measuring of SNR SNRNE_(B) ^(Phase2) and a signalor PSD PSD_(NErec-B)(f) at a first or near end of line B,

step f) comprises the measuring of SNR SNRFE_(B) ^(Phase2) and a signalor PSD PSD_(FErec-B)(f) at a second or far end of line B, and

step g) comprises determining FEXT transfer function according to

$\begin{matrix}{{{H_{AB}(f)}}^{2} = {\frac{{PSD}_{{FErec} - B}(f)}{P_{A}(f)} \cdot \lbrack {\frac{1}{{SNRFE}_{B}^{{Phase}\; 1}(f)} - \frac{1}{{SNRFE}_{B}^{{Phase}\; 2}(f)}} \rbrack}} & (5) \\{{{G_{AB}(f)}}^{2} = {\frac{{PSD}_{{NErec} - B}(f)}{P_{A}(f)} \cdot \lbrack {\frac{1}{{SNRNE}_{B}^{{Phase}\; 1}(f)} - \frac{1}{{SNRNE}_{B}^{{Phase}\; 2}(f)}} \rbrack}} & (6)\end{matrix}$

When using a DSL compliant test signal which covers the DSL upstream anddownstream sub channels, the NEXT/FEXT transfer function for all thesesub channels may be determined.

When using for example the ITU-T G992.1/3/5 REVERB, SEGUE or similarsignal as a test signal, many or all sub channels may be examined at thesame time which results in an enormous decrease of the measuring timecompared to using only one carrier at the same time. Otherwise the stepsa) to g) are repeated for all frequencies used in a DSL communication asa subcarrier frequency.

According to a further aspect of the invention, the tests are carriedout on all of the line pairs of interest in a manner that in a firstcycle line A remains the same while line B is selected sequentially fromthe remaining lines and in a second cycle line A is replaced with one ofthe remaining lines in sequence.

According to another aspect of the invention, the tests are carried outon all of the line pairs of interest in a manner that in a first cycleline A remains the same while all of the remaining lines as line B areselected in parallel for performing the measurement steps and in asecond cycle line A is replaced with one of the remaining lines insequence.

According to a still further aspect of the invention, the tests arecarried out on all of the line pairs of interest in a manner that atleast two line pairs are tested at the same time using test signals onlines A with non overlapping frequencies or frequency ranges. Whenperforming measurement on different line pairs in parallel, the overallmeasuring time can be further decreased, if measurement results forpossibly non-used frequencies are estimated by means of interpolation.

In order to increase accuracy of the measurements, the measurements canbe repeated and the FEXT/NEXT transfer function can be determined bymeans of averaging.

It is also possible with the method of the invention that themeasurements are performed and reported on at least one line B by meansof the ITU-T G.992.3 and G.992.5 Loop Diagnostic (DELT) or similar linetest protocols.

Further objects and advantages of the invention will be described inmore detail with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of a DSL system with the main componentsused in the method,

FIG. 2 is a schematic diagram of the measurement of FEXT/NEXT transferfunction on a pair of communication lines,

FIG. 3 is a schematic diagram of the steps during determining theFEXT/NEXT transfer function,

FIG. 4 is a timing diagram for carrying out the measurement on twocommunication lines,

FIG. 5 is a timing diagram for carrying out the measurement on severalcommunication lines in a sequence,

FIG. 6 is a timing diagram for carrying out the measurement on severalcommunication lines in parallel, and

FIG. 7 is a timing diagram for carrying out the measurement on severalcommunication line pairs in parallel.

DETAILED DESCRIPTION OF THE INVENTION

Referring first to FIG. 1, a typical DSL (digital subscriber line)system can be seen with a communication line between a CO (centraloffice) modem and a CPE (customer premises equipment) modem. Forexample, according to ITU-T G.992.1 Annex A the communication on an ADSLline is separated into voice communication in a frequency band up to 4kHz and into two digital communication bands for upstream datacommunication (25 kHz to 138 kHz) and downstream data communication (138kHz to 1104 kHz). These communication channels are divided into N=255sub channels with centre frequencies being a multiple of 4,3125 kHz alsocalled subcarriers. The use of sub channels or bins will be achieved bya transmission system using the principles behind Orthogonal FrequencyDivision Multicarrier (OFDM). In ITU-T G.992.1/3/5 and G.993.1/2, themodulation method is called Discrete Multi-Tone (DMT) and is similar toOFDM. Using DMT is useful since it allows the communications equipment(user modem and central office modem) to select only bins which areusable on the line thus effectively obtaining the best overall bit ratefrom the line at any time. With DMT, a combined signal containing manyfrequencies (for each bin) is transmitted through the line in bothupstream and downstream direction. Fast Fourier Transform (FFT) andInverse Fast Fourier Transform (IFFT) are used to convert the signal onthe line into the individual bins. The bits in each bin are encoded byQuadrature Amplitude Modulation (QAM) in order to increase Signal toNoise Ration and reduce transmission errors. During start-up the DMTsystem measures the Signal to Noise Ratio of the individual sub channelsand assigns different numbers of bits to each subchannel (carrier) inorder to maximize performance. This process is known as adaptivebitloading. A subchannel with low SNR is assigned a small number of bitsor no bits and a subchannel with a high SNR is assigned many bits. Thistechnique is robust in a typical DSL scenario where the line conditionsare unknown and slowly time-varying. Digital communication systems withhigher bitrates are proposed by ITU-T G.992.3 (ADSL2) with bitrates upto 12 Mbit/s and G.992.5 (ADSL2+) with bitrates up to 24 Mbit/s. ITUG.993.1/2 describes very high speed digital subscriber line transceivers(VDSL/VDSL2) with bitrates up to more than 50 Mbit/s.

A communication line is typically an unshielded twisted pair of copperwires used in telephone cables, however it may for example also be ashielded wire, a coaxial cable, or an optical cable. Communicationcables used in a DSL system enable a bidirectional communication with afrequency division system and/or echo cancelled system and a multicarrier modulation as described above.

FIG. 2 illustrates the principle structure for carrying out measurementsfor determining the NEXT/FEXT transfer function. A DSL modem typicallyconsists of a transmitter and a receiver connected to the subscriberline's twisted pair through a so called hybrid circuit. On the CO sidethe near end transmitters and receivers TR-AN and TR-BN and on the CPEside the far end transmitters and receivers TR-AF and TR-BF are shown.Both transmission lines Line A and Line B are of similar structure, e.g.unshielded twisted pair of copper wires. One of the transmission lines,for example Line A, is the “active” line. The central office CO side ofthe active line is connected to a first transmitter TR-AN whichgenerates a test signal that is applied to the first line Line A in afirst measuring cycle or phase 1. While transmitting the test signalS_(A) through the first line the second line is free of any test orcommunication signal. A noise measurement is than carried out on thissecond passive or quiet line indicated as Line-B at both ends of thecable. After obtaining a noise value QLNNE on the near end or CO sideand a noise value QLNFE on the far end or CPE side, these values arereported to a central control unit. The central control unit receivesthe measured values, makes the necessary calculations and coordinatedthe whole process of measurement.

In a second measuring cycle or phase 2 the test signal S_(A) is switchedoff so that the first line Line A is “silent”. While both the first lineand the second line are silent, that is free of any test orcommunication signal, a noise measurement is carried out on the secondpassive or quiet line indicated as Line B at both ends of the cable.After obtaining a noise value QLNNE on the near end or CO side and anoise value QLNFE on the far end or CPE side, these values are reportedto a central control unit.

In a third cycle or phase 3 the NEXT and FEXT transfer functions will bedetermined by the central control unit which performs the necessarycalculation on the received and stored values of the preceding noisemeasurements. After having determined the NEXT and FEXT transferfunctions, the line is ready for DSL service.

If only one subcarrier frequency is used in the test signal at a time,then the measurement cycles phase1 to phase3 have to be repeated for allsubcarrier frequencies. Therefore it is very advantageous to use a socalled reverb signal, a multi-carrier signal, which contains all thesubcarrier frequencies at the same time with substantially the sameamplitude but with a different phase, which remains constant duringmeasurement. This means that the received signal will contain a socalled line-spectrum, or in other words, the carriers remain orthogonalto each other which is very beneficial for this application.

Different measuring scenarios are illustrated in FIGS. 5 to 7.

According to one aspect of the invention the steps (phase 1 to phase 3)for determining the FEXT/NEXT transfer function are performed for amultiple of lines B1 to Bn in sequence while line A remains the same andthen a new Line A is selected. (FIG. 5) In this case only twocommunication lines are excluded from normal traffic during determiningthe FEXT/NEXT transfer function.

According to another aspect of the invention, the steps (phase 1 tophase 3) for determining the FEXT/NEXT transfer function are performedfor a multiple of lines B1 to Bn in parallel while line A remains thesame and then a new line A is selected. (FIG. 6) This example provides avery short measuring cycle, however during measuring all communicationlines A and B1 to Bn are excluded from traffic.

According to a third aspect of the invention, the steps (phase 1 tophase 3) for determining the FEXT/NEXT transfer function are performedfor a multiple of line pairs A1 B1 to An Bn in parallel with a testsignal S_(A1) to S_(An) wherein the test signals are signals with nonoverlapping frequencies or frequency ranges. (FIG. 7) This way theefficiency of the measurement may be further increased by means ofinterpolation, but again all communication lines under test are excludedfrom normal traffic.

In another embodiment of the invention, the steps (phase 1 to phase 3)for determining the FEXT/NEXT transfer function are repeated severaltimes and an average of the determined FEXT/NEXT transfer functions isgenerated.

It is of further advantage to use test signals and measuring meansdefined by standards. Line tests can be carried out according to thesingle ended line test SELT or the double ended line test DELTspecification.

Double Ended Line Test (DELT) measurement consists of a standardizedbroadband measurement procedure, specified by ITU-T and referred to asLoop Diagnostics. Both the far-end and the near-end modems requiresupport for Loop Diagnostics.

The DELT subscriber line measurement is performed from both ends of thecable for the purpose of DSL qualification, fault localization anddiagnosing.

The following parameters are measured per frequency (subcarrier) fromboth ends of the cable, i.e. upstream and downstream.

Hlin: The ITU-T standard G.992.3/G.992.5 defines a complex-valuedchannel transfer function H(f), measured per subcarrier.

Hlog: defined in ITU-T G.997.1 as the logarithmic representation of thetransfer function in dB.

Qin: Quiet Line Noise PSD per subcarrier is defined in ITU-TG.992.3/G.992.5 as the noise present in a particular subcarrier when noDSL service signal is present. The unit of this parameter is dBm/Hz.More specifically, this signal contains the following sub-signals

-   -   QLN-FE: QLN measured at the far-end side, i.e. customer side    -   QLN-NE: QLN measured at the near-end side, i.e. central-office        side

Snr: Signal-to-Noise-Ratio per subcarrier is defined in ITU-TG.992.3/G.992.5 as the ratio between the received signal and thereceived noise for a particular subcarrier (dB). The performance andreliability of the Loop Diagnostics parameters depend on theimplementation of Loop Diagnostics in both the near-end and the far-endmodem. This signal contains the following sub-signals

-   -   SNR-FE: SNR measured at the far-end side, i.e. customer side    -   SNR-NE: SNR measured at the near-end side, i.e. central-office        side

Using the DELT protocol for determining the FEXT/NEXT transfer function,the process may be even more effective.

FEXT-Transfer Function

The typical effective DSL noise environment is mainly determined by thecombination of natural background noise and FEXT, For a specificfrequency f and for a specific line j this can be described byσ_(j,eff) ²(f)=σ_(j,0) ²(f)+σ_(j,FEXT) ²(f)

The FEXT noise on line j is the accumulation of the individual crosstalkcontribution from the other lines

${\sigma_{j,{FEXT}}^{2}(f)} = {\overset{N}{\sum\limits_{{i = 1},{i \neq j}}}{{P_{i}(f)} \cdot {{H_{ij}(f)}}^{2}}}$

The number of FEXT-disturbers is N−1. Here, P_(i)(f) denotes thetransmit power (spectral density) on line i and |H_(ij)(f)|² denotes the(squared magnitude of the) FEXT-cross-transfer function from line i toline j.

Practically, there is no difference made between the differentcontributions to noise σ_(eff) ²(f). For the sake of readability theexplicit frequency dependence notation is omitted in the followingdescription.

The procedure for the determination of the FEXT-transfer function fromline A to line B is as follows:

-   -   1. Make sure line A and B are down, i.e. stop data transmission.        This is only necessary while performing DELT and to be able to        transmit defined signals on line A.        Phase 1    -   2. Start transmitting a signal with predefined PSD on line A.        Note that by utilising a Reverb-like signal, as standardized in        ITU-T G.992.1, the procedure is simplified and is automatically        in line with the DELT protocol. Practically, by using instead        the show time signal on line A the data traffic can be        uninterrupted, but lower measurement accuracy is expected. This        could however be compensated for by averaging via repetition of        step 3.    -   3. Perform DELT on line B.    -   4. When DELT has finished, retrieve DELT-QLN information,        denoted as QLNFE_(B) ^(Phase1), about QLN-FE on line B, i.e. the        measured QLN at the customer side.        Phase 2    -   5. Stop sending a signal on line A.    -   6. Perform DELT on line B.    -   7. When DELT has finished, retrieve DELT-QLN information,        denoted as QLNFE_(B) ^(Phase2), about QLN-FE on line B, i.e. the        measured QLN at the customer side.        Phase 3

All necessary measurements are now done and both line A and B are fullyavailable for active DSL service.

The calculation of the FEXT-transfer function will now be described. Theway DSL services are used makes FEXT-crosstalk properties, as forexample the PSD, only slowly changing over time.

Due to this fact the difference between the measurements of QLNFE_(B)^(Phase1) and QLNFE_(B) ^(Phase2) represents the (controlled) FEXTcontribution from line A to line B.

Since the PSD P_(A) of the defined transmit signal used on line A isknown, the crosstalk transfer function can be immediately calculatedfrom the difference of QLNFE_(B) ^(Phase1) and QLNFE_(B) ^(Phase2). Thatis

Δ QLN = QLNFE_(B)^(Phase 1) − QLNFE_(B)^(Phase 2) = P_(A) ⋅ H_(AB)²${H_{AB}}^{2} = \frac{\Delta\;{QLN}}{P_{A}}$

Note that this calculation is done for each used frequency (DSL-tones).The described procedure should be repeated for all line-pairs (A-B) inorder to characterize all the crosstalk-transfer functions of thebinder.

The practical accuracy of the estimation of |H_(AB)|² using theprocedure described above is increased with a decrease of the permanentnoise level, e.g. if all lines (except A and B) are not transmittingdata, i.e. they are silent. In practice, this is possible to conduct fora larger number of lines during regular maintenance sessions or duringthe nights. This minor drawback is well motivated and compensated by thepotential gains of providing this information to a DSM-based system orother expert systems aiming at optimized deployment.

NEXT-Transfer Function

The estimation of NEXT-transfer function is similar to the proceduredescribed above in connection with the FEXT transfer function. Here, theaccumulated NEXT-PSD on line j can be expressed as

${\sigma_{j,{NEXT}}^{2}(f)} = {\overset{N}{\sum\limits_{{i = 1},{i \neq j}}}{{P_{i}(f)} \cdot {{G_{ij}(f)}}^{2}}}$

Where the number of NEXT-disturbers is N−1, P_(i)(f) denotes thetransmit power (spectral density) on line i and |G_(ij)(f)|² denotes the(squared magnitude of the) NEXT-cross-transfer function from line i toline j.

In this case, we will utilize the QLN-NE information measured during thesame DELT-sessions as for the FEXT-case. Hence, the NEXT-transferfunction between line A and line B can be obtained by

Δ QLN = QLNNE_(B)^(Phase 1) − QLNNE_(B)^(Phase 2) = P_(A) ⋅ G_(AB)²${G_{AB}}^{2} = \frac{\Delta\;{QLN}}{P_{A}}$

The method uses standardized protocols already implemented in thesystems to conduct and retrieve the NEXT/FEXT-measurement results. Themethod is independent of the practically existing and small differencesbetween the sampling clock frequencies of the used lines. The method isadaptive to natural changes in line characteristics caused bytemperature and aging effects, etc. The method can be implementedentirely as a software solution as well. A special advantage arises fromthe fact that the suggested method does not need any additional ormodified equipment in a DSL system, the installed system componentscompliant with DSL standard can be programmed in order to perform therequired measurements and transfer the result of measurement from thelocation of measurement to the place of data processing. The FEXT/NEXTtransfer function is determined according to the invention in anautomatic way and provides real time information for crosstalksuppression methods and systems.

In another embodiment of the invention the calculation of NEXT and FEXTtransfer functions is based on SNR rather than only on Quiet Line Noise(QLN). In Phase 1 with a test signal on line A and Phase 2 without thetest signal on line A the following SNR are measured by the far-endreceiver of line B.

${{SNRFE}_{B}^{{Phase}\; 1}(f)} = \frac{{PSD}_{{FErec} - B}(f)}{{\sigma_{{{FE} - B},{eff}}^{2}\;(f)} + {{P_{A}(f)} \cdot {{H_{{AB}\;}(f)}}^{2}}}$${{SNRFE}_{B}^{{Phase}\; 2}(f)} = \frac{{PSD}_{{FErec} - B}(f)}{\sigma_{{{FE} - B},{eff}}^{2}(f)}$

Where PSD_(FErec-B)(f) is a received signal PSD for frequency f at thefar-end receiver side of line B. Assuming PSD_(FErec-B)(f) is the samein the both phases, after straight forward manipulations one ends upwith

$\begin{matrix}{{{{H_{AB}(f)}}^{2} = {\frac{1}{P_{A}(f)} \cdot \lbrack {{{\sigma_{{{FE} - B},{eff}}^{2}(f)} \cdot \frac{{SNRFE}_{B}^{{Phase}\; 2}(f)}{{SNRFE}_{B}^{{Phase}\; 1}(f)}} - {\sigma_{{{FE} - B},{eff}}^{2}\;(f)}} \rbrack}}{{{H_{AB}(f)}}^{2} = {\frac{{PSD}_{{FErec} - B}(f)}{P_{A}(f)} \cdot \lbrack {\frac{1}{{SNRFE}_{B}^{{Phase}\; 1}\;(f)} - \frac{1}{{SNRFE}_{B}^{{Phase}\; 2}\;(f)}} \rbrack}}} & ( {{Eq}.\mspace{14mu} 1} )\end{matrix}$

The NEXT transfer function |G_(AB)(f)|² can be calculated in a similarway by using the near-end measurement results instead of the far-endvalues. The involved measured signals or values are all available forexample from a DELT-procedure, but could also be measured by othermeans.

1. A method of determining a far-end crosstalk/near-end crosstalk(FEXT/NEXT) transfer function for a first line A and a second line B ina bundle of communication lines, said method comprising the steps of: a)transmitting at least during measuring intervals, an input test signalS_(A) at a near end of line A, said test signal having a known powerspectrum density (PSD) covering a frequency range of interest; b)measuring a first noise related measurement, PSD QLNNE_(B) ^(Phase1), ata first or near end of line B; c) measuring a second noise relatedmeasurement, PSD QLNFE_(B) ^(Phase1), at a second or far end of line B;d) stopping transmission of the test signal on line A, and thereafter:e) measuring a third noise related measurement, PSD QLNNE_(B) ^(Phase2),at a first or near end of line B; f) measuring a fourth noise relatedmeasurement, PSD QLNFE_(B) ^(Phase2), at a second or far end of line B;and g) determining the FEXT/NEXT transfer function on the basis of thefirst, second, third, and fourth measurements, wherein step g) comprisesdetermining the FEXT transfer function according to: $\begin{matrix}{{H_{{AB}\;}}^{2} = \frac{{QLNFE}_{B}^{{Phase}\; 1} - {QLNFE}_{B}^{{Phase}\; 2}}{P_{A}}} & (1)\end{matrix}$ and determining the NEXT transfer function according to:$\begin{matrix}{{G_{AB}}^{2} = \frac{{QLNNE}_{B}^{{Phase}\; 1} - {QLNNE}_{B}^{{Phase}\; 2}}{P_{A}\;}} & (2)\end{matrix}$ where P_(A) is the known PSD on line A; wherein eachmeasurement is reported to a central control unit and all steps arecoordinated by the central control unit.
 2. The method as recited inclaim 1, wherein the communication lines are Digital Subscriber Lines(DSL), and the method includes selecting an input test signal which isDSL compliant covering the DSL upstream and downstream sub channels. 3.The method as recited in claim 2, wherein the step of selecting an inputtest signal includes selecting a test signal to be a REVERB or a SEGUEsignal.
 4. The method as recited in claim 1, wherein: steps a) to g) arerepeated for all frequencies used in a Digital Subscriber Line (DSL)communication as a subcarrier frequency.
 5. The method as recited inclaim 1, wherein steps a) to g) are performed for a multiple of lines B1to Bn in sequence while line A remains the same, and then selecting anew line A.
 6. The method as recited in claim 1, further comprising thesteps of: h) repeating steps a) to g) for determining the FEXT/NEXTtransfer function several times; and i) generating an average of thedetermined FEXT/NEXT transfer functions.
 7. The method as recited inclaim 1, wherein the first, second, third, and fourth measurements areperformed and reported on at least one line B by means of the ITU-TG.992.3 and G.992.5 Loop Diagnostic (DELT) line test protocols.
 8. Amethod of determining a far-end crosstalk/near-end crosstalk (FEXT/NEXT)transfer function for a first line A and a second line B in a bundle ofcommunication lines, said method comprising the steps of: a)transmitting at least during measuring intervals, an input test signalS_(A) at a near end of line A, said test signal having a known powerspectrum density (PSD) covering a frequency range of interest; b)measuring a first noise related measurement, SNR SNRNE_(B) ^(Phase1), ata first or near end of line B; c) measuring a second noise relatedmeasurement, SNR SNRFE_(B) ^(Phase1), at a second or far end of line B;d) stopping transmission of the test signal on line A, and thereafter:e) measuring a third noise related measurement, SNR SNRNE_(B) ^(Phase2),and a noise signal, PSD σ_(NE-B,eff) ²(f), at a first or near end ofline B; f) measuring a fourth noise related measurement, SNR SNRFE_(B)^(Phase2), and a noise signal, PSD σ_(FE-B,eff) ²(f), at a second or farend of line B; and g) determining the FEXT/NEXT transfer function on thebasis of the first, second, third, and fourth measurements, wherein stepg) comprises determining the FEXT transfer function according to:$\begin{matrix}{{{H_{AB}(f)}}^{2} = {\frac{1}{P_{A}(f)} \cdot \lbrack {{{\sigma_{{{FE} - B},{eff}}^{2}(f)} \cdot \frac{{SNRFE}_{B}^{{Phase}\; 2}(f)}{{SNRFE}_{B}^{{Phase}\; 1}(f)}} - {\sigma_{{{FE} - B},{eff}}^{2}(f)}} \rbrack}} & (3) \\{{{G_{AB}(f)}}^{2} = {\frac{1}{P_{A}(f)} \cdot \lbrack {{{\sigma_{{{NE} - B},{eff}}^{2}(f)} \cdot \frac{{SNRNE}_{B}^{{Phase}\; 2}(f)}{{SNRNE}_{B}^{{Phase}\; 1}(f)}} - \sigma_{{{NE} - B},{eff}}^{2}} \rbrack}} & (4)\end{matrix}$ wherein each measurement is reported to a central controlunit and all steps are coordinated by the central control unit.
 9. Amethod of determining a far-end crosstalk/near-end crosstalk (FEXT/NEXT)transfer function for a first line A and a second line B in a bundle ofcommunication lines, said method comprising the steps of: a)transmitting at least during measuring intervals, an input test signalS_(A) at a near end of line A, said test signal having a known powerspectrum density (PSD) covering a frequency range of interest; b)measuring a first noise related measurement, SNR SNRNE_(B) ^(Phase1), ata first or near end of line B; c) measuring a second noise relatedmeasurement, SNR SNRFE_(B) ^(Phase1), at a second or far end of line B;d) stopping transmission of the test signal on line A, and thereafter:e) measuring a third noise related measurement, SNR SNRNE_(B) ^(Phase2),and a signal or PSD_(NErec-B)(f) at a first or near end of line B; f)measuring a fourth noise related measurement, SNR SNRFE_(B) ^(Phase2),and a signal or PSD_(FErec-B)(f) at a second or far end of line B; andg) determining the FEXT/NEXT transfer function on the basis of thefirst, second, third, and fourth measurements, wherein step g) comprisesdetermining the FEXT transfer function according to: $\begin{matrix}{{{H_{AB}(f)}}^{2} = {\frac{{PSD}_{{FErec} - B}(f)}{P_{A}(f)} \cdot \lbrack {\frac{1}{{SNRFE}_{B}^{{Phase}\; 1}(f)} - \frac{1}{{SNRFE}_{B}^{{Phase}\; 2}(f)}} \rbrack}} & (5) \\{{{G_{AB}(f)}}^{2} = {\frac{{PSD}_{{NErec} - B}(f)}{P_{A}(f)} \cdot \lbrack {\frac{1}{{SNRNE}_{B}^{{Phase}\; 1}(f)} - \frac{1}{{SNRNE}_{B}^{{Phase}\; 2}(f)}} \rbrack}} & (6)\end{matrix}$ wherein each measurement is reported to a central controlunit and all steps are coordinated by the central control unit.
 10. Amethod of determining a far-end crosstalk/near-end crosstalk (FEXT/NEXT)transfer function for a first line A and a second line B in a bundle ofcommunication lines, said method, comprising the steps of: a)transmitting at least during measuring intervals, an input test signalS_(A) at a near end of line A, said test signal having a known powerspectrum density (PSD) covering a frequency range of interest; b)measuring a first noise related measurement or PSD at a first or nearend of line B; c) measuring a second noise related measurement or PSD ata second or far end of line B; d) stopping transmission of the testsignal on line A, and thereafter: e) measuring a third noise relatedmeasurement or PSD at a first or near end of line B; f) measuring afourth noise related measurement or PSD at a second or far end of lineB; and g) determining the FEXT/NEXT transfer function on the basis ofthe first, second, third, and fourth measurements; wherein eachmeasurement is reported to a central control unit and all steps arecoordinated by the central control unit; and wherein steps a) to g) areperformed for a multiple of lines B1 to Bn in parallel while line Aremains the same and then selecting a new line A.
 11. A method ofdetermining a far-end crosstalk/near-end crosstalk (FEXT/NEXT) transferfunction for a first line A and a second line B in a bundle ofcommunication lines, said method comprising the steps of: a)transmitting at least during measuring intervals, an input test signalS_(A) at a near end of line A, said test signal having a known powerspectrum density (PSD) covering a frequency range of interest; b)measuring a first noise related measurement or PSD at a first or nearend of line B; c) measuring a second noise related measurement or PSD ata second or far end of line B; d) stopping transmission of the testsignal on line A, and thereafter: e) measuring a third noise relatedmeasurement or PSD at a first or near end of line B; f) measuring afourth noise related measurement or PSD at a second or far end of lineB; and g) determining the FEXT/NEXT transfer function on the basis ofthe first, second, third, and fourth measurements; wherein eachmeasurement is reported to a central control unit and all steps arecoordinated by the central control unit; and wherein steps a) to g) areperformed for a multiple of line pairs A1-B1 to An-Bn in parallel with atest signal SA1 to SAn wherein the test signals are signals withnon-overlapping frequencies or frequency ranges.
 12. The method asrecited in claim 11, wherein the step of determining the FEXT/NEXTtransfer function includes estimating the NEXT/FEXT transfer functionutilizing interpolation on non-used frequencies.